muon (symbol μ⁻) Negatively charged elementary particle, originally thought to be a meson but now classified as a lepton. It has spin ¹/₂, a mass c.207 times that of the electron, and decays weakly into an electron, a neutrino, and an antineutrino. See also antimatter

mu·on / ˈmyoōˌän/ • n. Physics an unstable subatomic particle of the same class as an electron (a lepton), but with a mass around 200 times greater. Muons make up much of the cosmic radiation reaching the earth's surface.DERIVATIVES: mu·on·ic / myoōˈänik/ adj.